Lithography is widely recognized as one of the key steps in the manufacture of integrated circuits (ICs) and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of ICs. In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Current lithography systems project mask pattern features that are extremely small. Dust or extraneous particulate matter appearing on the surface of the reticle can adversely affect the resulting product. Any particulate matter that deposits on the reticle before or during a lithographic process is likely to distort features in the pattern being projected onto a substrate. Therefore, the smaller the feature size, the smaller the size of particles critical to eliminate from the reticle.
A pellicle is often used with a reticle. A pellicle is a thin transparent layer that may be stretched over a frame above the surface of a reticle. Pellicles are used to block particles from reaching the patterned side of a reticle surface. Although particles on the pellicle surface are out of the focal plane and should not form an image on the wafer being exposed, it is still preferable to keep the pellicle surfaces as particle-free as possible.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are typically configured to output a radiation wavelengths of around 5-20 nm, for example, 13.5 nm or about 13 nm or 6.5-6.8 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
Some EUV sources, especially plasma sources, emit radiation over a wide range of frequencies, even including infrared (IR), visible, ultraviolet (UV) and deep ultraviolet (DUV) radiation. Radiation of these unwanted frequencies will propagate and cause heating problems in the illumination and projection systems and cause unwanted exposure of the resist if not blocked. Although the multilayer mirrors of the illumination and projection systems are optimized for reflection of the desired wavelength e.g., 13 nm, they have quite high reflectivities at IR, visible and UV wavelengths. As the resist to be exposed to the EUV radiation at the substrate is also sensitive to the non-EUV radiation like the DUV radiation, and the non-EUV radiation at the substrate does not contain information of mask pattern features. Instead, the presence of non-EUV radiation at the wafer stage only contributes to contrast loss. As such, it is desirable to keep the ratio of non-EUV radiation to EUV radiation below a certain value, which may be 1% at the substrate, just for example.
It has been proposed to use a filter to perform this function, such as a membrane-like spectral purity filter. However, such a filter is very delicate and has a limited heat load capability, leading to high thermal stresses and cracking, sublimation and oxidation in the high power levels of radiation occurring in a lithographic projection apparatus. A membrane filter also generally absorbs a significant portion of the desired EUV radiation. For example, DUV radiation may be suppressed by factor greater than 100 at the expense of 30% EUV radiation loss.
It has also been proposed to use a DUV-suppressing coating on one or more mirrors in the illumination and projection systems of a lithographic apparatus to perform this function. However, by using the DUV coating, the EUV radiation loss as compared to the DUV suppression is worse than that of using a membrane filter. Also, the approach of using a DUV-suppressing coating is insufficient as the reduction of the DUV to EUV ratio is limited to about a factor of 3.